1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing it, and particularly to a technology for fixing a semiconductor chip large in the amount of heat generated thereby to a support plate without having to use lead. The present invention relates to, for example, a technology effective for application to the manufacture of a low voltage-driven power transistor brought into low electrical resistance, which is used for a power supply or the like for a portable device or the like, a low thermal-resistance power transistor used for a power supply or the like for a high output device such as a laser beam printer, a large-current power transistor used for automobile electrical equipment or the like, etc.
2. Description of the Related Art
As a power transistor built in a charger for a cellular telephone, a video camera, etc. and a power circuit for an office automation (OA) device or the like, there is known a low voltage-driven power transistor using low on-resistance. For example, the low voltage-driven power transistor has been described in the xe2x80x9cHitachi Databook: Hitachi Semiconductor Packagexe2x80x9d issued on September, 1997, P.329, which has been published by Semiconductor Business Department of Hitachi, Ltd.
On the other hand, solder, a conductive adhesive, a silver paste or the like has been used as a bonding material for fixing a semiconductor chip to a chip fixing portion such as a support substrate or the like. As the conductive adhesive, there is known an adhesive containing conductive noble metal particles, a synthetic resin and an organic solvent as disclosed in Japanese Patent Application Laid-Open No. Hei 11(1999)-66956.
Further, a high conductive silver paste disclosed in U.S. Pat. No. 5,391,604 (Feb. 21, 1995) is known as the silver paste. The high conductive silver paste has been described in the xe2x80x9cElectronic Industry Materialxe2x80x9d published by Institute for Industrial Research, the July issue in 1997, P49-P54.
In a power device large in the amount of generated heat, solder containing lead has heretofore been used heavily as a bonding material for fixing a semiconductor chip to a mounting portion of a support plate. However, there has recently been a (leadless) move afoot to control the use of lead corresponding to a heavy metal from the viewpoint of the prevention of environmental destruction. However, the selection of a suitable material (adhesive) substitutable for lead-tin solder used for the conventional power transistors (package transistors called TO3P, TO220, etc.) remains a problem.
A structure of a power transistor will now be described in brief with reference to FIG. 16.
In a semiconductor device (power transistor) 1, a semiconductor chip 7 is fixed to its corresponding upper surface of a metal support plate 3 with a bonding material 13 interposed therebetween. The semiconductor chip 7 is fixed thereto so as to shift to the left of the support plate 3. The upper surface of the support plate 3 is covered with a sealing body or material (package) 2 in a state in which the right end of the support plate 3 is being exposed by a predetermined length. Thus, the lower surface (back) of the support plate 3 is exposed without being covered with the sealing material 2.
A plurality of leads 5 extending over the inside and outside of the sealing material 2 are provided at a left-end portion of the sealing material 2. These leads 5 constitute gate and source leads. Further, the support plate 3 serves as a drain.
The leads 5, which protrude from the end surface of the sealing material 2, are respectively bent downward in one stepwise form in the course thereof so as to take surface-mounted structures. Flat portions placed substantially in the same height as the tip support plate 3 serve as junctions. The leading ends of the leads 5 extending within the sealing body 2 and unillustrated electrodes of the semiconductor chip 7 are connected to one another by conductive wires 14.
Such a conductive adhesive and high conductive silver paste as described in the above references are considered as candidate materials each taken for an adhesive for fixing the semiconductor chip 7 to the support plate 3. It has been revealed that a problem arises in compatibility between high conductivity and reliability as a result of discussions about this point of view.
Namely, in a semiconductor device employed in various pieces of electronic equipment, a wide variety of functions have recently been formed within a semiconductor chip with advances in a system LSI, and the efficient outgoing radiation of heat generated within the semiconductor chip becomes an important subject for study. Upon chip bonding of the LSI, a silver paste has heretofore been used as a conductive adhesive containing no lead. This is an adhesive having conductivity even from either electrical or thermal viewpoint. Since the principal use of the conventional semiconductor device is intended for IC and LSI relatively low in power consumption, the electrical resistance of the conductive adhesive ranges from about 5 xcexcxcexa9cm to 1000 xcexcxcexa9cm and reaches a level increased by 100 times as compared with a metal junction.
FIG. 17 is a graph showing the general relationship between the content of a silver filler contained in a silver paste and resistivity thereof. In this silver paste, e.g., such a silver paste [hereinafter called a xe2x80x9cconductive adhesive (1)xe2x80x9d] as disclosed in Japanese Patent Application Laid-Open No. Sho 60(1985)-170658, which is brought into paste form by mixing and dispersing conductive powder and a liquid resin component, it is generally difficult to uniformly mix and disperse fillers with an increase in silver filler content, and a problem arises in terms of the implementation of resistivity equivalent to a metal. As indicated by the graph of FIG. 17, the conductive adhesive (1) has a silver filler content of about 85 Wt % even at the maximum.
In a silver paste [hereinafter called a xe2x80x9cconductive adhesive (2)xe2x80x9d] brought into paste form by mixing and dispersing a silver filler and a particulate resin component and using a volatile solvent as disclosed in U.S. Pat. No. 5,391,604, the silver filler and the resin particles are mixed and kept in a state floating in a medium in the case of the paste state. Therefore, a sliver filler content of 90% or more can easily be obtained as indicated by the graph of FIG. 17, and a significant reduction in electrical resistance value can be implemented as compared with the conductive adhesive (1).
Thus, such a silver paste that the silver filler content is brought to 85% or less as in the case of the conductive adhesive (1), is called a low conductive silver paste (whose heat conductivity is normally given as about 3 W/mxc2x7K) below. Such a silver paste that the silver filler content is high like 90% as in the case of the conductive adhesive (2), is also called a high conductive silver paste (whose heat conductivity is 60 W/mxc2x7K at maximum).
FIG. 18 is a characteristic diagram where the conductive adhesive (1) and the conductive adhesive (2) are respectively used as adhesives for chip bonding. Namely, the drawing shows a change in thermal resistance by a temperature cycle test at the time that a semiconductor chip with a power transistor built therein is mounted on a TO3P package. The same graph shows even one subjected to chip bonding with the conventional lead/tin solder for comparison.
According to such a result, it is understood that the conductive adhesive (2) greatly increases in thermal resistance with an increase in temperature cycle. This can be assumed to take place due to a reduction in resin component in the silver paste, an increase in small bubbles formed upon curing of the silver paste and degradation in bonding performance.
Further, the conductive adhesive (1) does not increase in thermal resistance upon the temperature cycle test. This means that the bonding strength of an adhesive for bonding a semiconductor chip to a support plate is high. Accordingly, the adhesive like the conductive adhesive (1) is called a high bonding or junction strength adhesive.
Thus, the high conductive silver paste [conductive adhesive (2)] is excellent in that the resistivity is low as compared with the low conductive paste (high junction strength adhesive) like the conductive adhesive (1). It is difficult to say that the high conductive silver paste is preferable to the low conductive silver paste in terms of an increase in thermal resistance in a temperature cycle test on a power product.
An allowable limit for the thermal resistance will not be demonstrated here because it differs for each product. However, it is a reality that the lower the thermal resistance and electrical resistance, the better. Characteristics commonly equivalent or close to the conventional lead/tin solder are required for power products large in the amount of generated heat, such as a subsequent power transistor, etc.
An object of the present invention is to provide a technology of fixing a semiconductor chip to a support plate by a highly thermal conductive adhesive and a high junction strength adhesive without having to use lead corresponding to an environmental harmful substance.
Another object of the present invention is to provide a semiconductor device stably operated even relative to the heat generated upon its operation, and a method of manufacturing it.
A further object of the present invention is to provide a technology of manufacturing a semiconductor device which makes no use of an environmental harmful substance.
The above, other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.
Summaries of typical ones of the inventions disclosed in the present application will be described in brief as follows:
(1) A semiconductor device wherein a semiconductor chip is fixed onto a support plate with an adhesive interposed therebetween, has a structure wherein highly thermal conductive adhesives each containing no lead corresponding to an environmental harmful substance, and a high junction strength adhesive are used so as to separate bonding areas from one another. Namely, there is provided a semiconductor device comprising a support plate which supports a semiconductor chip, and the semiconductor chip fixed onto the support plate with an adhesive interposed therebetween, and wherein the semiconductor chip is fixed to the support plate by a first adhesive and a second adhesive provided so as to separate bonding areas from one another, and the first adhesive is a highly thermal conductive adhesive higher in thermal conductivity than the second adhesive and the second adhesive is a high junction strength adhesive higher in junction strength than the first adhesive.
As a specific structure, there is provided a semiconductor device (TO220 type or the like) comprising a sealing body comprised of an insulative resin; a metal support plate which has at least part covered with the sealing body, and a lower surface exposed from the sealing body and which serves as a first electrode; a suspender lead which is continuously connected to the support plate and protrudes from one side of the sealing body; a second electrode lead which serves as a second electrode, and a control electrode lead which serves as a control electrode, both of which protrude from the one side of the sealing body side by side; and a semiconductor chip which is covered with the sealing body and has a first electrode at a lower surface thereof, and which has a second electrode and a control electrode at an upper surface thereof and whose lower surface is fixed to the support plate with a conductive adhesive interposed therebetween; and wires which are placed within the sealing body and respectively electrically connect the second electrode and the second electrode lead, and the control electrode and the control electrode lead. The semiconductor chip has a whole fixing area thereof fixed to the support plate by at least one highly thermal conductive adhesive and a high junction strength adhesive provided so as to separate bonding areas from one another. No space gap exists in an interface between the highly thermal conductive adhesive and the high junction strength adhesive. The highly thermal conductive adhesive is provided in plural places within the whole fixing area, and the partial highly thermal conductive adhesive is placed so as to correspond to a heated portion of the semiconductor chip. The highly thermal conductive adhesive and the high junction strength adhesive contain no lead. The semiconductor chip is provided with power MOSEETs with the first electrode, second electrode and control electrode respectively set as electrodes.
According to the means of (1) referred to above, (a) the heat generated from the semiconductor chip can be transferred to the support plate through the highly thermal conductive adhesive with high efficiency and hence heat-dissipated. Further, the semiconductor chip can firmly be fixed to the support plate by the high junction strength adhesive. As a result, stable heat radiation can be carried out without an increase in thermal resistance even in the case of a temperature cycle test. It is thus possible to improve a radiation characteristic and an electrical characteristic and achieve a stable operation of the semiconductor device.
(b) The semiconductor chip can firmly be fixed to the support plate without having to use an environmental harmful substance.
(c) Even in the case of the semiconductor device such as the TO220 type or the like, a malfunction caused by the heat generated from the semiconductor chip does not take place and hence a stable operation can be achieved.